Is the buzzing at the box on the wall where the up/down buttons are, or at the motor? I believe that unit has a magnetic clutch that could be the issue, but if the buzzing is at the electrical box, it could be as simple as dirty contacts in the push-button making the motor-run relay chatter.
One thing you may be able to do to isolate the problem: when you push the button for "down," does it instantly engage, no buzz? That would indicate that the problem is the "up" button. Of course, you may be past the down limit switch (if there is one), in which case the down button won't do anything at all.